It is believed that embodiments of the invention described herein materially enhance the quality of the environment of mankind by contributing to the restoration or maintenance of one or more basic life-sustaining natural elements, including air, water, and/or soil. The invention and certain embodiments thereof are more fully described below.
Exhaust, Industry, and Pollution
Engines produce much of the power and mechanical work used across the globe. The internal combustion engine (ICE) is the most common engine type. The ICE burns fuel within a confined space to generate motion and power. The ICE is ubiquitous, being found in motor vehicles, locomotives, air- and marine-craft, tractors, generators, power plants, manufacturing facilities, industrial equipment and the like. Fuels used to power ICEs include gasoline, natural gas, diesel, ethanol, and vegetable oil. Inherent inefficiencies in both engine mechanics and the fuels themselves result in incomplete burning of fuel, leading to the production and emission of various pollutants. Thus, while a great innovation and convenience, the millions of engines currently in use represent a substantial source of air pollution.
The pollution produced by ICEs may be characterized as being either particulate or nonparticulate. Particulate pollution is generally characterized by the presence of small solid and/or liquid particles, such as carbonaceous soot and ash, dust, and the like. Nonparticulate pollutants include gases such as carbon monoxide, oxides of nitrogen and sulfur, ozone, and the like, as well as unburned hydrocarbons and volatile organic compounds. Particulate pollutants can be filtered from the exhaust and either discarded or further oxidized into less egregious gaseous products. Nonparticulate pollutants may likewise be converted to nonpollutants. Significant nonpollution emissions from ICEs include nitrogen gas (N2), carbon dioxide (CO2), and water vapor (H2O). These emissions are generally benign to humans (although excess levels of atmospheric CO2 are believed to contribute to global warming).
Air pollution poses a serious health risk to people and can damage the environment. Ozone (O3), for example, is a respiratory irritant and can cause coughing, irritation in the throat, reduction of lung function, feelings of chest tightness, wheezing, shortness of breath, and the aggravation of asthma. Ozone may also contribute to the formation of smog. Likewise, microscopic solid particles or liquid droplets may work their way deep into the lungs; these particles may be irritants or, worse, carcinogens. When exposed to such particles, people may also experience nose and throat irritation, bronchitis, pneumonia, as well as increased risk of heart and/or lung disease, headaches, nausea, allergic reactions, chronic respiratory disease, lung cancer, heart disease, neurological damage, and damage to organs such as the liver and kidneys. Continual exposure to air pollution may have adverse effects on the lungs of growing children and may aggravate or complicate medical conditions in the elderly.
Medical conditions arising from exposure to air pollution may be very expensive to the individual. Further, in aggregate these medical conditions represent a drain on the economy, both in terms of direct loss of productivity as well as in tax dollars spent on healthcare. Such expenses amount to billions of dollars annually. Reducing the adverse health effects of pollution by reducing the amount of airborne pollutants would not only enhance the overall respiratory health of the population but would also decrease the substantial burden borne by the healthcare system and the drain on the economy as a whole.
To this end, government and industry have committed to reducing the level of air pollution. Government agencies set emissions standards and implement pollution regulations. The Kyoto Protocol is an example of inter-governmental cooperation in calling for worldwide reductions in greenhouse gases.
The chief U.S. regulatory agency is the Environmental Protection Agency (EPA), created by the 1970 amendments to the Clean Air Act (CAA) of 1963. The CAA is the comprehensive Federal law that regulates air emissions from area, stationary, and mobile sources. The EPA has set regulations for various pollutants. These “criteria pollutants” include: (1) ozone (O3); (2) lead (Pb); (3) nitrogen dioxide (NO2); (4) carbon monoxide (CO); (5) particulate matter (PM); and (6) sulfur dioxide (SO2). Further, a 1990 amendment to the CAA was made to address problems such as acid rain, ground-level ozone, stratospheric ozone depletion, and air toxics. As a result, the EPA issued 175 new regulations to reduce automotive emissions, gasoline reformation, the use of ozone depleting chemicals, and the like.
Ground-level ozone is a primary constituent of smog. Ozone is generally not directly emitted into the air, but instead is formed by the reaction of volatile organic compounds (VOCs) or reactive organic gases (ROGs) with nitrogen oxides (NOx) in the presence of heat and sunlight. VOCs/ROGs are emitted from a various sources, including ICE combustion byproducts, solvents, petroleum processing, and pesticides. Nitrogen oxides are likewise produced as ICE combustion products. Since 1997, the national ambient air quality standards for ozone are an 8-hour 0.08 ppm standard.
Nitrogen dioxide (NO2) is a reactive gas that can be formed by the oxidation of nitric oxide (NO). Nitrogen oxides (NOx) play a major role in the formation of ozone and smog. The major sources of man-made NOx emissions are high-temperature combustion processes.
Carbon monoxide (CO) is a colorless, odorless, and potentially deadly that can be produced by the incomplete combustion of carbon in fuels. Motor vehicle exhaust contributes about 60% of CO emissions in the U.S. In cities, as much as 95% of the CO in the air is generated by automobile exhaust.
Particulate matter includes microscopic airborne solid particles and/or liquid droplets. PM can be further classed as “fine” (particles have diameters of less than 2.5 microns, hereinafter ‘PM-2.5’) and “coarse” (particles have diameters in excess of 2.5 micrometers). Fine particles result primarily from fuel combustion in motor vehicles, power generation, industrial facilities, and from residential fireplaces and wood stoves. Coarse particles 107 are typically generated by such sources as vehicles traveling on unpaved roads, materials handling equipment, and crushing and grinding operations. (See FIG. 5). Some PM is emitted directly from the source (such as smokestacks and automobiles) while other PM is generated through the interaction of gases such as SO2, NOx, VOC and other airborne compounds to form fine particles. The chemical and physical compositions of the particles vary.
Sulfur dioxide can be formed when fuel containing sulfur (such as coal and oil) is burned, for example, during metal smelting and other industrial processes.
ICEs are directly effected by these regulations, since the engines emit both criteria pollutants and air toxics. ICEs typically run on one of two types of petroleum fuel, either gasoline or diesel. Each type of fuel contains complex mixtures of hydrocarbon compounds as well as traces of many other materials, such as sulfur. While other types of fuel can also be used, such as ethanol/petroleum fuel mixtures, vegetable oils, and the like, all known fuels produce some degree of emissive pollution when burned. In addition, lubricants are often added to the fuel mix to enhance engine performance and durability.
In order to reduce emissions, ICEs are increasingly designed to carefully control the amount of fuel burned. In gasoline engines, the air-to-fuel ratio is maintained very close to the stoichiometric point, the calculated ideal ratio of oxygen to fuel. At the stoichiometric point, all of the fuel introduced into the engine will theoretically be burned using all of the oxygen available from the air introduced into the engine. In practice, the fuel-to-air ratio actually varies from the ideal stoichiometric ratio quite a bit during driving, since the oxygen content of the air is not necessarily a constant, air density fluctuates with temperature (which is also not a constant), the engine valve settings may vary with temperature, vibration and impact effects are random, and the like. Sometimes the mixture is lean (the air-to-fuel ratio is relatively high) and other times the mixture is rich (the air-to-fuel ratio is low). These deviations tend to be uncontrolled and thus result in the generation of various pollutants.
Motorcycles represent another type of mobile, on-road vehicle and include both two and three-wheeled motorcycles designed for on-road use. Motorcycles primarily use gasoline fuel. Emissions control technologies for motorcycles include conversion of 2-stroke engines to 4-stroke, advanced injection systems (injection timing, injection pressure, rate shaping, common rail injection, and electronic controls), pulse air systems, changed combustion chamber design (higher compression ratios, piston geometry, and injector location), and the use of catalytic converters. Limitations in motorcycles' emissions control technologies are different than those in light or heavy-duty vehicles. With motorcycles, the focus is more on the appearance, placement, and heat generation and removal of aftertreatment devices, as there are fewer places to “hide” aftertreatment devices and the passenger is in much closer proximity to the exothermic oxidation reaction. Additionally, the vibratory accelerations experienced in motorcycles by the aftertreatment devices are generally more damaging in motorcycles than in automobiles.
Diesel engines also contribute to the criteria pollutants. These engines use hydrocarbon fractions that auto-ignite when compressed sufficiently in the presence of oxygen. In general, diesel combusting within a cylinder produces greater amounts of particulate matter and the pollutants nitrogen and sulfur oxides (NOx and SOx respectively) than does gasoline combustion. Even so, diesel mixtures are generally lean, with relatively abundant amounts of oxygen present. Consequently, the combustion of the smaller hydrocarbons is usually more complete, producing less carbon monoxide than gasoline. Longer chain hydrocarbons are more difficult to burn completely and typically result in the formation of significant amounts of particulate residues, such as carbon “soot.”
Due to the importance of improving air quality and complying with relevant laws and regulations, substantial time, money and effort have been invested in developing technologies for reducing engine emissions. Emission reduction technologies may be classified as follows: engine improvements, fuel improvements, and after-treatments. These classifications are for convenience and are not mutually exclusive. Engine improvements technologies include advanced injection systems, exhaust gas recirculation, electronic sensors and fuel controls, combustion chamber designs, advanced turbocharging, and variable valve timing. Fuel improvement technologies include fuel formulations, such as high octane, low aromatic, low sulfur, fuel borne catalysts, liquefied petroleum gas (LPG), oxygenation of fuels, compressed natural gas (CNG) and biodiesels. After-treatment technologies include catalytic converters (2, 3, and 4-way), particulate traps, selective catalytic reduction, NOx adsorbers, HC adsorbers, NOx reduction catalysts, and many others. Some systems incorporate various pieces of these and other technologies. For example, catalyzed diesel particulate traps traverse the categories as set forth above.
One notable emission control device is the catalytic converter. Catalytic converters operate by providing a site where the potential barrier for the conversion of a pollutant species into a nonpollutant species is significantly lowered. Catalytic converters enjoy widespread use in cars and light-duty vehicles. Recent improvements in converters (such as somewhat increased substrate geometric surface area, lower thermal mass, optimized washcoating, reduced catalyst loading but higher reactive surface area, etc.) have yielded incremental improvements in emissions control. To meet the increasingly strict requirements imposed automobiles, manufacturers will likely increase catalyst loading or the number of substrates per vehicle. Other emissions control technologies include advanced injection systems (injection timing, injection pressure, rate shaping, common rail injection, electronic controls), exhaust gas recirculation, changes in combustion chamber design (higher compression ratios, piston geometry, and injector location), advanced turbo charging, conventional catalytic converters, and catalytic exhaust mufflers.
Diesel engine emissions control technologies are similar to those used on gasoline vehicles and include advanced injection systems (injection timing, injection pressure, rate shaping, common rail injection, electronic controls), exhaust gas recirculation, changes in combustion chamber design (higher compression ratios, piston geometry, and injector location), advanced turbo charging, ACERT, diesel particulate filters, NOx adsorbers, DPNR systems, selective catalytic reduction, fuel reformers, fuel-borne catalysts, catalytic exhaust mufflers, and diesel oxidation catalysts. Exhaust gas recirculation (EGR) has been problematic due to its tendency increase the generation of particulate pollution. EGR also requires cooling of the recirculated gasses, which necessitates a larger radiator, and thus a larger nose on the vehicle, creating aerodynamic and fuel economy constraints.
Exhaust gas recirculation (EGR) directs a portion of the exhaust gas stream back into the air intake of the engine. The recirculated exhuast gas has a higher specific heat and can absorb some of the heat generated during combustion, hence lowering the temperature of combustion. EGR does not require regular maintenance and works well in combination with high swirl-high turbulence combustion chambers. EGR drawbacks include reduced fuel efficiency and engine life, greater demands on the vehicle's cooling system, limitation to decreasing only NOx, emissions and a requirement of control algorithms and sensors. Thus, there is a limit to the amount of EGR that can be applied to any engine, and EGR is often used in parallel with another control technology.
The technology used to control emissions from stationary sources varies widely, but examples include filters, scrubbers, sorbents, selective catalytic reduction (SCR), precipitators, zero-slip catalysts, catalysts for turbines, or oxidation catalysts.
Reformulating or using different fuels is another emissions control technique applicable to both mobile and stationary engines, since some fuels inherently pollute more than others and other fuels tend to degrade catalysts that could otherwise function to clean the exhaust air. For instance, although the shift from leaded to unleaded fuel was driven be the degradatory effect of lead on catalytic converters, the removal of lead from fuels also greatly decreased lead emissions. Lowering the sulfur content in fuel reduces SOx emissions and increases the efficiency of many catalytic converters, as sulfur can also degrade catalysts.
Use of aftertreatment devices to remove pollutants from post-combustion exhaust gasses is very common. Perhaps the most common aftertreatment device is the catalytic converter. Catalytic converters designs vary widely, but in general function to remove combustible pollutants from the exhaust stream via catalysis of further combustion reactions. The composition and placement of the catalyst(s), and the substrate(s) used to support them, vary with converter design.
A two-way catalytic converter performs oxidation of gas-phase pollution, such as the oxidation of HC and CO to CO2 and H2O. While two-way converters are effective at controlling HC and CO and require little maintenance, they can actually increase NOx emissions and tend to be degraded by lead and sulfur.
A three-way catalytic converter performs both oxidation (conversion of CO and HC to CO2 and H2O) and reduction (conversion of NOx to N2 gas) reactions. Further performance improvements by these devices are limited by a number of factors, such as the temperature range and surface area of their substrates and by the susceptibility of the catalyst to further degradation.
Particulate traps or filters are another type of aftertreatment device commonly used in diesel applications, as the combustion of diesel fuel generates a considerable amount of particulate matter. In a diesel particulate trap (DPT), also sometimes called the diesel particulate filter or soot filter, the exhaust stream is directed through a filter for collection of airborne particles. The removal of accumulated particulate matter from the trap at a later time is commonly referred to as “regeneration” and can be accomplished in a number of ways. One method of regeneration applies externally generated heat to “burn off” the trapped particulates. Alternately, small amounts of diesel fuel may be released into the exhaust stream to ignite upon contact with the filter to raise the filter temperature sufficiently to completely burn the trapped particulate matter. Still alternately, fuel borne catalysts may be supplied to facilitate regeneration. Yet alternately, the filter may include a catalyst to reduce the temperature necessary for the PM to burn off (“catalyzed diesel particulate filter or catalyzed soot filter (CSF)”). Still alternately, an oxidation catalyst may be placed upstream of the filter to facilitate burn off of the PM. Diesel particulate traps can reduce PM emissions by as much as 85%. Traps utilizing a catalyst can reduce the emissions of other, non-PM pollutants (e.g., HC, CO, and PM) for even cleaner emissions. Diesel particulate traps include a number of different types of filter configurations, including, powder ceramic monolithic, fiber-wound, knitted fiber, woven fiber, sintered metal fiber and filter paper, among others. The drawbacks of particulate traps include impedance of the exhaust stream when the traps become clogged with particulates (soot, ash, and the like) thus increasing exhaust back pressure and making the engine work harder. Further, some trapped particulates, such as ash, may degrade catalysts used in the catalyzed versions. Some traps also cannot survive the large thermal gradients experienced during regeneration. Moroever, particulate traps inherently add cost and weight to vehicles.
Selective catalytic reduction (SCR) is another example of an aftertreatment system for reducing the amount of NOx in the exhaust stream. In SCR, a chemical capable of acting as a reducing agent (such as urea) is added to the exhaust stream before the exhaust reaches the catalyst chamber. The reducing agent hydrolyzes to form ammonia, which then reacts with NOx in the exhaust gas to reduce the NOx to yield nitrogen (N2) gas, thereby decreasing NOx emissions. Alternately, ammonia may be directly injected in to the exhaust stream. An oxidation catalyst is often used in parallel with SCR to reduce CO and HC. Unfortunately, while SCR is effective in reducing NOx with low catalyst deterioration and good fuel economy, it requires an additional tank for the reducing agent and an infrastructure for refilling the tank and dispersing the agent. SCR is also dependent on end user compliance (i.e., keeping the reducing agent supply filled) in order to maintain the emissions control.
NOx adsorbers are materials that store NOx under lean conditions and release and catalytically reduce the stored NOx under fuel rich conditions (typically every few minutes). This technology is viable in both gasoline and diesel applications, and some engines provide a better fuel rich, high temperature environment. NOx adsorbers reduce the levels of HC, NOx, and CO, but have little to no effect on PM. NOx adsorbers have the advantage of functioning under a wide range of temperatures. Conversely, NOx adsorbing capacity decreases with decreasing temperature and thus requires the installation of engine controls and sensors. Further, NOx adsorbers tend to be degraded by high sulfur content in fuel. There are additional constraints in diesel applications, including the quantity of oxygen present in the exhaust, the HC utilization rate, the temperature range, and smoke or particulate formation.
An NOx reduction catalyst can also be used to control emissions. This is accomplished by actively injecting reductant into the system upstream of the catalyst and/or using a washcoat (such as with zeolite) that adsorbs HC and thus creates an oxidizing region conducive to reducing NOx. While this technology can reduce NOx and PM emissions, it is relatively expensive and can lead to poor fuel economy and/or the generation of sulfate particulates.
HC adsorbers are designed to trap VOCs while cold and then release the VOCs when heated. HC adsorbers may be coated directly onto the catalytic converter substrate, allowing for minimal system changes but less control. Alternately, the HC adsorbers may be located in a separate, but connected, exhaust pipe upstream of the catalytic converter with a valve in place for redirecting the exhaust stream once the converter is heated. Still alternately, the HC adsorbers may be positioned downstream of the catalytic converter. The last two options require a cleaning option for the adsorber. While this technology reduces cold start emissions, it is difficult to control and adds cost.
Since no technology yet exists to single-handedly reduce all types of emissions, individual emissions control technologies are often combined in an exhaust system. Examples of combination systems include a DeNOx and DPT combination, a catalytic converter placed in the muffler, SCR integrated with the muffler, a catalyzed diesel particulate filter, and the like. Another example of a system incorporating multiple emissions control technologies is ACERT, which targets four areas—intake air handling, combustion, electronics, and exhaust aftertreatment. Key components include single and series turbocharging for cooling intake air, variable valve actuation for improving fuel burns, electronic multiplexing for integrating computer control, and catalytic conversion for reducing tailpipe particulate emissions. Working in concert, these subsystems allow for an increase in fuel efficiency.
Catalytic Converters
The catalytic converter operates to change some of the pollutants in the exhaust stream into less harmful compounds, commonly occurring molecules such as N2, H2O, and CO2. Basically, the catalytic converter provides a surface on which reactions are encouraged that convert pollutants into the relatively harmless reaction products mentioned above.
Typical pollutants in combustion exhaust include nitrogen oxides (NOx), unburned hydrocarbons, carbon monoxide, and the particulate matter. The nitrogen oxides can be reduced to form nitrogen. When a NO or NO2 molecule contacts an appropriate catalyst material, the catalyst facilitates removal of nitrogen from the molecule, freeing oxygen in the form of O2. Nitrogen atoms adhering to the catalyst then react to form N2 gas: 2 NO=>N2+O2 and 2 NO2=>N2+2 O2. The carbon monoxide, unburned hydrocarbons, and particulate matter can be further oxidized to form relative nonpollutants. For example, carbon monoxide is reacted as shown: 2 CO+O2=>2 CO2. The overall result of the catalytic converter is to substantially complete the combustion of fuel into relative nonpollutants.
Conventional catalytic converters have a number of limitations to their effectiveness of eliminating pollutants. For example, if the converter is located too close to an engine, it may crack from thermal stresses brought about by rapid local heating. Accordingly, the filters used in conventional catalytic converters are typically not placed immediately adjacent or inside an engine exhaust manifold, an otherwise optimal location to take advantage of the in situ high exhaust gas temperatures. Moreover, engine vibration and large temperature gradients near and within the exhaust manifold can cause conventional filter material to degrade and fatigue, substantially decreasing the life of the filters. In addition, some catalysts applied to conventional filters work less efficiently or even cease to function at high temperatures, i.e., above 500 degrees Celsius. Accordingly, catalytic converter filters are usually positioned remote from the engine.
Structures of Catalytic Converters and Particulate Filters
The components of a catalytic converter are shown in FIGS. 1, 3 and 4. The catalyst substrate is held within the converter shell or canister by a packaging mat, typically made of ceramic fibers. The shell is fluidically connected to the exhaust stream, such that exhaust gasses generated by the engine are directed through the converter on their way to the atmosphere. Catalytic converters, especially those found in gasoline engine applications, may be also equipped with heat shielding to protect adjacent vehicle components from exposure to excessive temperatures.
Generally, a catalytic converter is composed of five main components: 1) a substrate; 2) a catalytic coating; 3) a wash-coat; 4) a matting; and 5) a housing canister.
Substrate
The substrate is a solid surface on which the pollutants can be converted to relative nonpollutants. Physically, a substrate provides the interface for several molecular species to react with each other. The substrate generally defines a large surface area to provide a platform on which pollutants may be converted to relative nonpollutants.
Typical substrate configurations include woven or pleated fibrous papers, honeycomb monoliths and beads. A honeycomb structure typically contains numerous channels, usually running parallel to each other along the length of the substrate, to provide high surface area for reactions. (See FIG. 1) These channels further function to direct the flow of exhaust gas from the engine through the catalytic converter and out through exhaust pipe. In the bead structure, the substrate is made of a collection of small beads filling an enclosure (similar to putting a bunch of jelly beans in a tube). The exhaust flows around the beads and the pollutants are converted to nonpollutants on the bead surfaces. Similarly, the woven fiber structure exposes a large amount of fiber surface area to the exhaust gas pollutants upon which reactions may occur. Substrate compositions include ceramic metal oxides, foamed ceramics/glasses, reticulated foams, power ceramics, nanocomposites, metals, and fiber mat-type substrates. The most commonly used substrate material is cordierite.
Catalytic Coating
Catalytic coating of the substrate provides the surface upon which the conversion of pollutants to non-pollutants may be done at reduced temperatures or with reduced energy input. The catalyst influences the rate of a chemical reaction but does not take part in the reaction, i.e., it is not consumed or altered in the reaction. In other words, catalysts facilitate reactions which are otherwise too slow or which would otherwise require high temperatures to be efficient. As used in catalytic converters, the catalysts allow more complete combustion of HC pollutants at lower temperatures.
Since catalysts provide the ability to enable or accelerate certain chemical reactions between exhaust gas components, solid catalysts are particularly useful in the catalysis of gas phase reactions. The catalytic effect is maximized by good contact between the gas phase and the solid catalyst. In catalytic reactors, this is usually realized by providing finely dispersed catalyst on high specific surface area substrate support.
The catalytic material is typically added to the substrate as a coating after formation. Different catalyst compositions are selected depending upon the chemical reaction to be catalyzed. Other factors influencing catalyst selection include the chemical environment, temperature conditions, economic factors (i.e., catalyst cost) and the like. A number of metal catalysts are known, the most common being platinum, palladium, rhodium and alloys of the same. As noble metal catalysts are both rare and expensive, much effort has gone into the development of other, less rare catalyst compositions.
The rates of chemical reactions, including catalyzed reactions, generally increase with increasing temperature. A strong dependency of conversion efficiency on temperature is a characteristic feature of almost all emission control catalysts. A typical relationship between the catalytic conversion rate of an exhaust gas constituent and the temperature is shown as the solid line in FIG. 2. The conversion rate, near-zero at low temperatures, increases slowly at first and then more rapidly with increasing reaction temperature, and reaches a plateau at high temperatures. When discussing combustion reactions, the term light-off temperature is commonly used to characterize this behavior. The catalyst light-off temperature is generally taken to be the minimum temperature necessary to initiate the catalytic reaction. More precisely, the light-off temperature may be taken to be the temperature at which conversion reaches 50%, or T50. When comparing the activities of different catalysts, the most active catalyst will be characterized by the lowest light-off temperature for a given chemical reaction.
In some catalyst systems, increasing the temperature may only increase the conversion efficiency up to a certain point, as illustrated by the dashed line in FIG. 2. Further temperature increase, despite increasing reaction rates, causes a decrease in the catalyst conversion efficiency. The declining efficiency is generally considered to arise from other competing reactions which deplete the concentrations of reactants, by thermodynamic reaction equilibrium constraints, and/or by thermal agitation of gas molecules at the catalyst surface preventing the gas molecules from contacting the catalyst long enough to be catalyzed.
The temperature range corresponding to the high conversion efficiency is frequently called the catalyst temperature window. This type of conversion curve is typical for selective catalytic processes.
Another important variable influencing the conversion efficiency is the size of the reactor. The gas flow rate through a catalytic reactor is commonly expressed, relative to the size of the reactor, as space velocity (SV). The space velocity is defined as the volume of gas, measured at standard conditions (STP), per unit time per unit volume of the reactor, as follows: (3) SV=V/Vr where V is the volumetric gas flow rate at STP, m3/h; Vr is the reactor volume, m3, and SV has the dimension of reciprocal time which is commonly expressed in 1/h or h−1.
Wash Coat
In most cases, the catalytic coating includes a wash coat. The washcoat is an intermediate layer applied to the surface of the substrate, thereby increasing its effective surface area and providing a surface to which the catalyst adheres. Metal catalyst may thus be impregnated on this porous, high surface area layer of inorganic carrier. Additionally, the washcoat can physically separate and prevent undesired reactions between components of a complex catalytic system. Washcoat materials include inorganic base metal oxides such as Al2O3, SiO2, TiO2, CeO2, ZrO2, V2O5, La2O3 and zeolites. Some of these materials primarily serve as catalyst carriers, while others are added to the washcoat as promoters or stabilizers. Still other washcoat materials exhibit catalytic activity of their own. Good washcoat materials are characterized by high specific surface area and thermal stability.
Canister
FIG. 3 illustrates a typical prior art catalytic converter. The substrate is packaged into a canister, such as a steel shell, to form a catalytic converter. The canister performs a number of functions, including housing the catalyzed substrate and protecting the substrate from the external environment. The canister may be dedicated to the converter or may perform another function, such as housing a muffler. Additionally, the canister directs the flow of exhaust gas through and/or over the catalyzed substrate. The catalyzed substrate is usually placed inside the canister having a configuration made according to one of several methods, including: clamshell, tourniquet, shoebox, stuffing, and swaging, as shown in FIG. 4.
Matting
In addition to the canister, a matting material is often used to package the catalytic substrate in the canister. The packaging mats, usually made of ceramic fibers, protects the substrate and evenly distribute the pressure from the shell. The mats often include vermiculite, which expands at high temperatures, thus compensating for the thermal expansion of the shell and providing adequate holding force under a wide range of operating conditions.
Heat Insulation
In many applications, the catalytic converter is heat insulated to avoid damage to surrounding vehicle components (e.g., plastic parts, fluid hoses) and/or to prevent an increase of engine compartment temperature. One method of managing the heat output of the converter is the employment of a refractory heat shield positioned around the converter body.
Particulate Trap
DPTs are relatively effective at removing carbon soot from the exhaust of diesel engines. The most widely used DPT is the wall-flow filter which filters the diesel exhaust by capturing the soot on the porous walls of the filter body. The wall-flow filter is designed to provide for nearly compete filtration of soot without significantly hindering the exhaust flow.
As the layer of soot collects on the surfaces of the inlet channels of the filter, the lower permeability of the soot layer cause a pressure drop across the filter and a gradual rise in the back pressure of the filter against the engine, causing the engine to work harder, thus reducing engine operating efficiency. Eventually, the pressure drop becomes unacceptable and removal of the trapped particulates from the filter (i.e., filter regeneration) becomes necessary. In conventional systems, the regeneration process involves heating the filter to initiate combustion of the trapped carbon soot. In certain circumstances, the regeneration is accomplished under controlled conditions of engine management whereby a slow burn is initiated and maintained for a number of minutes, during which the temperature in the filter rises from about 400-600° C. to a maximum of about 800-1200° C.
In certain applications, the highest temperatures reached during regeneration tend to occur near the exit end of the filter due to the cumulative effects of the wave of soot combustion that progresses from the entrance face to the exit face of the filter as the exhaust flow carries the heated combustion gasses down the filter. Further, a so-called “uncontrolled regeneration” can occur when the onset of combustion coincides with, or is immediately followed by, high oxygen content and low flow rates in the exhaust gas (such as engine idling conditions). During an uncontrolled regeneration, the combustion of the soot may produce temperature spikes within the filter that can thermally shock and crack, or even melt, portions of the filter. The most common temperature gradients observed are radial temperature gradients (wherein the temperature of the center of the filter is hotter than the rest of the substrate) and axial temperature gradients (wherein the exit end of the filter is hotter than the rest of the substrate).
In addition to capturing the carbon soot, the filter also traps metal oxide “ash” particles that are carried by the exhaust gas. Usually, these ash deposits are derived from uncombusted lubrication oil that sometimes accompanies the exhaust gas. These particles are not combustible and, therefore, are not removed during regeneration. However, if temperatures during uncontrolled regenerations are sufficiently high, the ash may eventually sinter to the filter or even chemically react with the filter, resulting in localized melting of the filter.
It is desirable to obtain a filter which offers improved resistance to melting and thermal shock damage so that the filter not only survives numerous controlled regenerations over its lifetime, but also (hopefully) survives the less frequent but more severe uncontrolled regenerations.
Continuous Regeneration Trap
Several conventional methods exist for controlling regeneration of DPTs. An application of catalyst to the filter can reduce the oxidation temperature of particulate matter. Likewise, the filter can be preceded with a chamber containing oxidation catalyst that creates NO2, to help burn off particulate matter. Also, utilize fuel-borne catalysts may be provided. Additionally, an external source of heat m ay be provided to burn off trapped soot and the like (with or without catalysts). In any event, regeneration leaves behind ash residue that requires maintenance to clean the filter.
Some filter designs include diesel oxidation catalysts (DOCs). DOCs catalyze the oxidation of CO and hydrocarbons. Hydrocarbon activity extends to the polynuclear aromatic hydrocarbons (PAHs) and the soluble organic fraction (SOF) of particulate matter. Catalyst formulations have been developed that selectively oxidize the SOF while minimizing oxidation of sulfur dioxide or nitric oxide. However, DOCs may produce sulfuric acid byproducts and increase the emission of NO2 as byproducts of the oxidation of hydrocarbons.
The main refractory component of conventional filters is a ceramic (typically cordierite or SiC) wall-flow monolith. The porous walls of the monolith are coated with an active catalyst. As the diesel exhaust aerosol permeates through the walls, the soot particles are deposited within the wall pore network, as well as over the inlet channel surface. The catalyst facilitates diesel particulate matter (DPM) oxidation by the oxygen present in exhaust gas.
Pressure Drop
The flow of exhaust gas through a conventional catalytic converter is accompanied by a substantial amount of backpressure. The management of backpressure buildup in a system equipped with a catalytic converter is important. If the catalytic converter is partially or wholly clogged, it will create a significant restriction in the exhaust system, resulting in a drop in engine performance (e.g., horsepower and torque) and thus decrease fuel economy and may even cause the engine to stall.
High filtration efficiencies of wall-flow filters are obtained at the expense of a relatively high pressure drop which increases with the accumulation of soot in the filter (soot load). As the soot and ash particles start depositing within the pores in the monolith walls (depth filtration) of a clean filter, the pressure drop across the filter increases non-linearly over time.
Limitations of Current Substrates
While catalytic converter and particulate matter filter technology is integral to the reduction of emission pollution, there still remain certain drawbacks. One important limitation of catalytic converter technologies is the relatively low melting temperature of cordierite, which limits the placement of cordierite devices in an engine system and makes the devices susceptible to melting during uncontrolled regeneration events. Accordingly, an improved substrate for use in a catalytic converter or particulate filter would be significant advance in the fundamental physical and chemical attributes of the materials used as catalyst substrates in the catalytic converter.
The conventional monolithic catalytic converter substrate is generally formed from a powder-based ceramic slurry through an extrusion process. The traditional extrusion process is limited as to how small the channels may be made within the material and still maintain quality control. This, in effect, places a limit on the geometric surface area that can be achieved in cordierite honeycomb substrates. The extrusion process also limits the shapes of the catalytic converters to cylinders or parallelograms, or shapes that have sides parallel to the extrusion axis. The honeycomb configuration is formed using an extrusion process in which long channels with their major axes parallel to the extrusion action are created. The opening of these channels faces the incoming exhaust airflow.
Decreasing the wall thickness increases the surface area by allowing for more walls per unit volume. For example, by decreasing wall thickness from 0.006 inches to 0.002 inches, a 54% increase in surface area may be achieved. By increasing the surface area, more particulate matter may be deposited in the same volume. FIG. 1 shows a prior art honeycomb configuration formed within a ceramic filter element configured to increase the surface area for a catalytic converter.
However, the physical limitations of this material have been approached. Because of the physical characteristics of ceramics, and, in particular cordierite, use of cordierite substrates with even thinner walls is not practical. The thinner-walled material is not able to meet other necessary characteristics (e.g., durability, heat resistance) requisite for survival in the operating conditions experienced by catalytic converters.
Diesel filters, in part because of their larger sizes, often have thicker walls than their automotive counterparts. Because diesel wall flow filters generally have thicker walls, there are physical limitations on the number of channels per square inch these filters can have. Generally, there are no commercially available diesel wall flow filters having more than 200 channels per square inch.
Another limitation of currently available substrates is their decreased catalytic efficiency at lower temperatures. When a converter system is cold, such as at engine start up, temperatures are not sufficiently high to commence the catalyzed reactions. The cordierite, silicon carbide, and various metal substrates employed in currently available catalytic converters are fashioned from very tough, dense materials with excellent mechanical strength and tolerances for thermal shock and vibration. However, these materials require a long time to absorb heat after start-up to reach temperatures sufficient for catalytic reactions. Due to the delay in the catalysis reaction start-up, it is estimated that approximately 50% of all of the emissions from modern engines are released to the atmosphere during the first 25 seconds of engine operation. Even a small improvement during these critical “cold start” seconds could drastically reduce the amount of pollutants emitted annually. While effort has been made to address this problem, there remains a need for a catalytic converter that can reduce emissions during this critical cold start period.
To more quickly achieve reaction temperatures, attempts have been made to move the converters closer to the engine exhaust manifold where higher temperatures are more quickly available and also serve to drive reactions more vigorously during operation. Because usable space near the engine of a vehicle is limited, the size of converter systems, and therefore the amount of throughput that can be successfully treated, is likewise limited. Current substrates cannot be effectively used in the very high temperature environments of the engine compartments of vehicles. Moreover, adding additional weight to the engine compartment is undesirable, and many current substrates are dense with limited porosity (roughly 30% or less), requiring systems that are both heavy and voluminous to treat large scale exhaust output. Additionally, substrates such as cordierite are susceptible to melting under many operating conditions, thereby causing clogs and increased back pressure.
Other methods of compensating for cold starts include elaborate adsorption systems to store NOx and/or hydrocarbons temporarily so that they might be treated once the converter has reached critical temperatures. Some of these systems require parallel piping and elaborate adsorption surfaces, additional valves and control mechanisms, or multiple layers of differing washcoats for adherence of catalysts to substrates and segregation of reaction environments. This problem is especially challenging in diesel engines where large volumes of soot particulates, NOx, and SOx may need to be trapped. In some large industrial diesel engines, rotating banks of diesel particulate traps are used to collect, store, and treat particles. (In still other systems, NOx is stored and used as an oxidizing agent to convert CO into CO2 while it is reduced to N2).
Another inherent limitation of conventional systems is the typical “regeneration time” required to burn off particulates. Given the large volume of exhaust gas throughput and the speed at which the gas must flow, a converter should be capable of rapid light off. Thus, a substrate capable of rapid burn-off/light-off, of enduring extreme thermal and vibrational shocks, and capable of rapid internal temperature build up during cold starts, is desirable. If the substrate were also lightweight, it would also result in improved mileage statistics on new vehicles. Thus, there remains a need for an improved lightweight, tough and durable catalyst substrate with a high melting point, high porosity and permeability, thermal shock resistance, and ability to hold a catalyst. The present invention addresses this need.